As well known from, for example, U.S. Pat. No. 3,812,735, a key-shift transmission is fashioned to have a speed-change shaft, a plurality of speed-change gears rotatably mounted on the speed-change shaft, a shifter sleeve slidably mounted on the speed-change shaft, and shift keys co-slidably connected to the shifter sleeve and operable to couple the speed-change gears one at a time to the speed-change shaft and permits multi-stage changes of the vehicle speed in a compact structure so that it is well suited for use in a small-sized working vehicle.
In the past, shift keys for such key-shift transmission have been disposed within elongated axial grooves in the outer surface of a speed-change shaft. For forming such axial grooves, a shaft blank having a relatively large outer diameter is employed which is then processed by broaching so as to form elongated axial grooves in the outer surface.
Such speed-change shaft is relatively large in weight and, therefore, disadvantageous in power transmission purpose. Material coast is relatively high and production coast is further enhanced due to machining for forming the axial grooves.